The use of horizontal trees is rapidly becoming the norm for subsea completions because of the cost reduction offered over conventional technology. As subsea completions enter deep water the cost saving increases dramatically up to 25% as reported in an article entitled "Horizontal Trees Provide Quick Well Bore Access, 25% Cost Savings", edited by Leonard Le Blanc, Offshore International Magazine November 1993. Horizontal wellheads also allow for larger completions to be utilised in conventional technology, allowing a field to be exploited by fewer wells. However, the conventional method of isolating the horizontal wellhead after workover and before production, which may be a period of several years, is by situating wireline plugs in the tubing hanger. This is viewed by some major oil companies as a less than ideal method. One particular problem with this is that it is very difficult and indeed dangerous to extract the plugs prior to production. It has frequently been found that such plugs are often very difficult to remove which can add considerably to the cost of the exercise.